I spotted this on eBay last week, listed as “U.S: USED #RC20a STRIP/4 CV $240+ UNLISTED AS STRIP”.

I’m not sure why RC20a isn’t listed as a strip of 4 when RC20 is listed as a strip. I hope to fix that when I submit my changes for the 2019 U.S. Specialized this summer. It doesn’t make sense to have one without the other.

Current catalog value as 4 singles for RC20a is $280 (4 x $70) so the starting price of $175 wasn’t exactly a bargain… but after some scrutiny I decided to place a snipe. I was the only bidder.

This was the only image in the listing, and at this resolution. What did I see that made me try to purchase it?

Adding some spoiler space…

There was just enough aberration in the strokes, stems, and serifs of the bottom stamp compared to the others, to make me think this might be a strip with the double transfer, which only ever occurs in the bottom stamp of the strip.

And so it is. It makes a perfect matching companion for the RC20 strip with DT I have. Both strips are shown below (new RC20a at left and RC20 at right), followed by a high resolution closeup of the DT stamp.